1. Field
The present disclosure relates to a developing device and an image forming apparatus.
2. Description of the Related Art
An electrophotographic system, which enables easy formation of high-quality images at relatively low-cost, has been widely adopted into image forming apparatuses such as a copier, a printer, and a facsimile. An electrophotographic image forming apparatus comprises: for example, a photoreceptor drum of which surface is provided with a photosensitive layer containing a photoconductive substance; a charging section for charging the surface of photoreceptor drum; an exposing section for exposing the surface of photoreceptor drum being charged, to form thereon an electrostatic latent image; a developing section for supplying a toner to the electrostatic latent image formed on the surface of photoreceptor drum, to form thereon a toner image; a transfer section for transferring to a recording medium, etc. the toner image carried on the surface of photoreceptor drum; and a fixing section for fixing the toner image existing on the recording medium by subjecting the toner image to heating, pressing, and the like operation. In the electrophotographic image forming apparatus as described above, a non-magnetic one-component developing device is generally used as the developing section in view of facilitation of maintenance, reduction in size and weight of the image forming apparatus itself, and thus of cost reduction. A commonly-used non-magnetic one-component developing device is composed of a developer tank, a developing roller, a supply roller, a stirring member, and a doctor blade. The developer tank accumulates a toner therein, and supports the developing roller, the supply roller, and the stirring member so as to be rotatable. The developing roller is disposed in pressure-contact with the photoreceptor drum, and has its surface carrying a toner layer. The supply roller is disposed in pressure-contact with the developing roller, and supplies a toner to a surrounding area of the developing roller. The stirring member stirs a toner accumulated in the developer tank. The doctor blade is disposed in contact with the surface of developing roller to regulate a layer thickness of the toner layer on the surface of developing surface. The non-magnetic one-component developing device serves to form a toner image by supplying a toner from the toner layer on the surface of developing roller to an electrostatic latent image on the photoreceptor drum.
The non-magnetic one-component developing device has a problem that a toner is more easily coagulated compared to the case of using a two-component developer containing a toner and a magnetic carrier. Especially in a contact portion between the developing roller and the doctor blade, the toner coagulation causes a serious problem. Since the doctor blade comes into contact with the surface of developing roller under pressure in order to regulate the layer thickness of the toner layer on the surface of developing roller, there is given relatively high pressure and frictional heat between the developing roller and the doctor blade. The toner coagulation resulting in generation of a toner mass at the contact portion leads decrease in flowability of the toner, in a consequence whereof the toner mass remains on the portion and is subjected to higher pressure and frictional heat which cause the toner mass to be fused and then adhered to the contact portion of the doctor blade with the developing roller. The fused toner adhered to the doctor blade scrapes the toner layer on the developing roller, which contacts the toner-adhered part, thus causing an image defect such as white stripes. Moreover, the pressure imparted to the toner-adhered part becomes higher, thus setting a condition on which the fusion and adhesion of toner is promoted more easily. Accordingly, the toner-adhered part grows in size over time and thereby gives flaws to the surface of developing roller, with the result that a service life of the apparatus itself may possibly be shortened.
Denaturing a surface property of the doctor blade in order to prevent the toner from being fused and adhered to the doctor blade has been proposed. For example, attentions are paid to the aspect that the toner is fused and adhered not to a contact portion of the doctor blade but a non-contact portion thereof with the developing roller. From the aspect, there has been proposed to apply synthetic resin having low-surface energy, such as silicone resin and fluorine resin, to the non-contact portion of the doctor blade with the developing roller (refer to Japanese Unexamined Patent Publication JP-A 62-86381 (1987), for example). Another proposal is to form on at least a surface of the contact portion of the doctor blade with the developing roller a nickel-phosphorus composite plated layer containing fluorine resin dispersed therein (refer to Japanese Unexamined Patent Publication JP-A 2001-194896, for example). The techniques disclosed in JP-A 62-86381 (1987) and JP-A 2001-194896 cannot, however, sufficiently prevent the toner from undergoing the fusion and adhesion because an image forming apparatuses of date uses a toner having a relatively low melting point. In addition, as stated in JP-A 62-86381 (1987), it is well known that the toner is not frequently fused and adhered to the contact portion but fusion and adhesion of the toner to the non-contact portion more frequently occurs.
Further, there has been a proposal of disposing a platy doctor blade such that one end in a transverse direction thereof is supported by a developer tank and the other end thereof is brought into contact with or spaced a distance away from a surface in a vertical direction of a developing roller, wherein the transverse direction extends in the vertical direction, and further providing a pressing member which slides in the vertical direction to rub a surface of the doctor blade positioned at upstream side of a rotation direction of the developing roller (refer to Japanese Unexamined Patent Publication JP-A 58-150984 (1983), for example). However, the fusion and adhesion of toner easily occur also on a surface of the doctor blade positioned at downstream side of the rotation direction of the developing roller, and actually in many cases, the fusion and adhesion of toner occurring on the downstream surface is massive. Accordingly, the fused and adhered toner cannot sufficiently removed by the pressing member. Another problem of JP-A 58-150984 (1983) is risk that the pressing member sliding in parallel with the surface of the doctor blade might cause the toner to be caught between the pressing member and the rubbing surface of the doctor blade, resulting in the toner adhesion over an entire surface of the doctor blade. Moreover, in JP-A 58-150984 (1983), it is stated that the pressing member is made to move up and down in the vertical direction to thereby rub the surface of the doctor blade. However, JP-A 58-150984 (1983) contains no disclosure about a mechanism of moving the pressing member up and down. It is thus very difficult to realize the invention described in JP-A 58-150984 (1983).